Electroless copper plating

ABSTRACT

By means of constant control of the temperature between 70* and about 80* C. and control of the cyanide ion concentration between 0.0002 and 0.0004 molar, high quality electroless copper plating processes are carried out from solutions containing cupric ions.

United States Patent [1 1 Underkofler et al.

[ Oct. 29, 1974 ELECTROLESS COPPER PLATING [75] Inventors: William L. Underkofler, Vestal;

Theodore D. Zucconi, Endicott, both of N.Y.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: Dec. 17, 1973 21 Appl. No.: 425,363

[52] US. Cl. 106/1, 117/160 [58] Field of Search 106/1 Primary ExaminerLewis T. Jacobs Attorney, Agent, or Firm-Joseph G. Walsh [5 7] ABSTRACT 7 Claims, N0 Drawings 1 ELECTROLESS COPPER PLATING FIELD OF THE INVENTION The present invention is concerned with a process for electroless copper plating. In particular it is concerned with a process for obtaining very high quality plating. Materials made according to the process of the present invention have such high quality that they are suitable, for example, in printed circuit applications where extremely high reliability is absolutely essential. In addition, the products made according to the process of the present invention have fine grained uniaxial metallurgical structure and can withstand the thermal shock of wave soldering without the cracking of surface lands or plated through holes. The process of the present invention is characterized by very careful control of the temperature and of the cyanide concentration of the plating bath.

PRIOR ART Electroless copper plating, per se, is well known in the prior art. However, electroless copper deposits previously known have been deficient in metallurgical properties which are required for high quality and high reliability. In particular, the prior art materials have had poor deposit structure. This structure has generally been a coarsely columnar one. The prior art materials have also had low ductility and large fissures between grain boundaries which readily open to cracks under mechanical stressing. In contrast, the materials made by the process of the present invention stand severe mechanical testing without electrical or mechanical failure by cracking. The tests passed include flexing, hostile environment, repetitive thermal cycling and solder shock and rework. Deposits plated under the specified conditions contain less hydrogen than in the prior art by an order of magnitude. Hydrogen in copper deposits ordinarily is adsorbed on grain boundaries, promoting mechanical weakness at the boundaries.

The prior art describes many electroless copper plating baths including several which, at first blush, appear quite similar to that used in the present invention. US. Pat. No. 3,403,035 in Example 4 thereof shows an electroless plating process carried out at 80C. in a bath containing cyanide ions. The concentration of the cyanide ions, however is 0.0000] molar, while for the process of the present invention, it is essential that the cyanide ion concentration be at least 20 times greater than that prior art value. This same reference, at line 62 of Column 4 does show a bath containing 0.000l moles per liter of sodium cyanide, or half of that required in the present invention. It is to be noted that the reference says that with this concentration, no copper was deposited. The present invention, therefore, goes directly against the teaching of this prior art reference, and uses an even higher cyanide ion concentration.

U.S.' Pat. No. 3,607,3l7 shows electroless plating processes carried out in the presence of cyanide ions but the temperature used is 45C. (in bath one) and 60C. (in bath 2). According to the process of the present invention, it is essential that the temperature be at least 70C.

The teachings of US. Pat. Nos. 3,6l5,737 and 3,635,758 both include suggestions that the temperature of an electroless plating bath may be between and 100C, usually between and 80C. Both of these patents, however, specifically teach that the temperature may vary widely. (The former patent teaches this at Col. 6, Line 33 and the latter at C0]. 5, line 44.) Thus, the process of the present invention goes directly contrary to the teaching of both of these prior art patents in that for the process of the present invention it is essential that the temperature be within the range from C to about C.

US. Pat. No. 3,095,309 teaches the preferred temperature for electroless copper plating to be from 40C to 50C (Column 5, line l5). Thus, the present invention is again contrary to the prior art.

SUMMARY OF THE INVENTION The present invention is concerned with a process for electroless copper plating to produce products of superior electrical and mechanical properties. The process is characterized by very careful control of the temperature and of the cyanide ion concentration of the electroless plating bath.

Electroless copper plating baths have been known in the past. They contain a source of cupric ion generally, cupric sulfate. They contain a reducing agent such as formaldehyde. They contain a complexing agent for the cupric ion, such as ethylenediamine tetracetic acid (EDTA). They contain a surfactant and in some cases, other minor additives. A satisfactory surfactant is, for example, an organic phosphate ester available under the trademark Gafac RE-6l0. The pH of the system is often controlled, for example, by the addition of sodium hydroxide in the desired amount.

It has now been found that vast improvement in the end product can be obtained in electroless plating processes when the temperature of the bath during the plating operation is controlled between 70 and about 80C. When the temperature is allowed to fall below 70C. cracking of the plated surface results. Very good results are obtained over the range of 70 77. The most preferred temperature is 73C. Above about 80, however, the process is not conveniently carried out due'to equipment limitations.

In the process of the present invention, it is also essential that the cyanide ion concentration be carefully controlled. The desired results are obtained when and only when the cyanide ion concentration is maintained during the plating operation at a value of from 0.0002 to 0.0004 molar. This corresponds to a concentration of from about 10 to about 20 milligrams per liter of sodium cyanide. When the concentration is allowed to go below 0.0002 molar, there is insufficient cyanide present to provide the required structural modification action. On the other hand, when the cyanide ion concentration is above 0.0004 molar the deposittends toward a coarsely columnar structure and metallurgical properties decline with the plated structure cracking under thermal shock of wave soldering.

It should particularly be noted that it is necessary that continuous control of temperature and cyanide ion concentration be maintained. The reason for this is not understood, but the empirical fact is that the bath operates with what might be called chemical hysteresis, i.e., if the conditions are allowed to. go out of the required range, and are then corrected back to within the required range, it will be several hours before sound deposits can be obtained again. The reason for this is not understood, but it is postulated that perhaps the sluggish reactions which are operated in the bath permit only slow approach to the required equilibrium.

The following Examples are given solely for purposes of illustration and are not to be considered limitations on the invention, many variations of which are possible without departing from the spirit or scope thereof.

tional electroplating (e.g., pyrophosphate) copper plating. The following table compares properties of a copper deposit produced by the process described immediately above and compares it with one produced by electroplating. It will be .noted that the present electroless' 5 EXAMPLE 1 plating process compares favorably with electroplating. The following bath and operating conditions are used to illustrate a preferred embodiment of the present 1nventton: k Formulation: Table I Ethylenedtamine tetraacetic acid dihydrate 30-50 g/l Cupric sulfate pcntahydrate 8-12 g/l Present Formaldehyde 0.7-2.2 g/I Property Invention Elcctroplated Sodium hydroxide to pH 11.6-11.8 Surfactant (1.2-0.3 g/l Ductility Elongation 0.8-1.072 1.2-1.35% Sodium cyanide 10-25 mg/l Tensile Strength" 35-40 Kpsi 35-45 Kpsi O eratin Conditions: Yield Strength -30 Kpsi 20-35 Kpsi lemperature 70-80C Hardness (Knoop 50) 70-90 55-75 Specific gravity* 1060-10110 Resistivity 1.80 X 10 ohm-cm 1.87 X 10 ohm-cm Aeration Continuous air bubbling Folding Endurance 19 cycles 22 cycles Stirring 10 times per hour turnover Hydrogen Content 0.62 c.c./gm Agitation Continuous agitation of plating racks 2O Plating rate 0.06-0.12 mil/hour Specific gravity incrcusesfthroughlhy-product buildup; It is controlled by bleeding v V V V oil" used bath and adding rcsh but on a continuous asis. 1 I "Plating rate is adjusted by controlling formaldehyde concentration within the followlng Table luustrqtes the cylucal effect of sou-d temperature on copper deposit properties:

Table 11 m CuSO -SH O (g/l) 10 10 10 10 10 7.5 EDTA (g/l) 35 35 35 35 35 35 Gafac RE-6l0 (g/l) 0.25 0.25 0.25 0.25 0.25 0.25 NaCN (mg/l) 15 16 15 15 3O 13 pH 11.70 11.70 11.70 11.70 11.70 11.70 HCHO (37%) (ml/l) 3.4 2.8 3.6 4.2 .5 3.0 Specific Gravity 1.065 1.065 1.065 1.065 1.065 1.065 Temp. (C) 73 71 76.5 6 9 73 Plating Rate (mil/hr) 095 .102 .103 .071 .054 .101 Percent Cracks 0 0 87 0 By means of the process such as that described The following Table illustrates the critical effect of above, there are obtained copper deposits having qual- 45 cyanide ion concentration on copper deposit proper- 1ty comparing favorably to that produced by conventies:

Table III Formulation CuSZL-5H 0 (g/l) 10 10 10 10 10 10 EDTA (g/l) 35 5 35 35 35 Gafac REE-610 (g/l) 0.25 0.25 0.25 0.25 0.25 0.25 Sodium cyanide (mg/l) 14 12 8 35 30 pH 11.70 11.70 11.70 11.70 11.70 11.70 HCHO (37%) (ml/l) 3.3 3.7 3.4 5.0 3.0 3.5 Specific Gravity 1.06 1.065 1.065 1.065 1.065 1.065 Temp. (C) 73 73 73 3 3 Results Plattng Rate (mil/hr) .047 .104 .082 .060 .055 .050 Percent Cracks 0 0 0 69 86 60 Other Variables Formulation u 2 (g/l) to 10 10-54 10 EDTA ('g/l) 35 35 35 Gafac RE-6l0 (g/l) 0.25 0.25 0.25 0.25 Sodium cyanide (mg/l) 17 11 15 16 all 11.7 11.73 11.70 11.70-11.0

(H0 (37%) (ml/I) 5.0 3.3 2.0 2.9 Specific Gravity 1.080 1.065 1.042 1.065 Temp. ("C) 73 73 73 73 Results Platlng Pate (mil/hr) .09 .08 .10 .073 Percent Cracks 0 0 0 0 A summary of the optimum and allowable ranges for 2. A process as claimed in claim 1 wherein the temthe plating conditions used in the process of the present perature is maintained within one half degree of 73C. invention is given in Table IV below. 3. A process as claimed in claim 1 wherein the cya- Table IV Bath Formulation Operating Parameters and Ranges Optimum Range Allowable Range PP?' u a M 1 Pentahydrate 9-l l g/l 7.5-12 g/l Formaldehyde (37%) 2.5-4 ml/l 2-4.5 ml/l Sodium cyanide 9-l5 mg/l 7-20 mg/l pH pH |1.70:0.02 11.70 :01 Ethylenediamine Tetraacetic Acid Dihydrate (EDTA) 35 i g/l 35 i 10 g/l Specific Gravity l.060l.070 (C) l.060l .080 (25C) Plating Rate 0.095 i 0.01 mil/hr 0.07-0.12 mil/hr Temperature 73 a: 0.5C 70C to 80C Bath Loading -150 Cm"'/| 30-200 Cm /l' Aeration Continuous Continuous Gafac RE-6l0 Wetting Agent 0.25 t 0.05 g/l 0.25 t 0.1 g/l Notes:

1. The optimum range defines best operating conditions, and is the process control objective. 2. The allowable range gives the permitted deviations from the optimum range for temporary deviations only. Corrective action is to be taken to return to the optimum range.

h i cl me i 2 Hide"ion co ncentratioii is m'aiaiaifiawery ass; to

1. An electroless copper plating process in which 0.00025 molar. copper is deposited from a bath solution comprising cu- 4. A process as claimed in claim 1 wherein the reducpric ions, a reducing agent, a surfactant, and a coming agent is formaldehyde. plexing agent for cupric ions, said process being char- 5. A process as claimed in claim I wherein the comacterized by the improvementacocording to which the 30 plexing agent is ethylenediamine tetraacetic acid. temperature of the 55th'fim'aiiiiaineddfiiifiuausw Kg." 6. process as claimed in claim 1 wherein the surfactwam c and so e aaathabam is aa us'eaama tam (game Phosphate estert r 7 t tib t5] 7. A process as claimed in claim 1 wherein the pH is 510.3 9P 3 93 91. 2 99 95.35? maintained within the range 11.6 to 1 L8. the range of 0.0002 to 0.0004 molar. 

1. AN ELECTROLESS COPPER PLATING PROCESS IN WHICH COPPER IS DEPOSITED FROM A BATH COMPRISING CUPRIC IONS, A REDUCING AGENT, A SURFACTANT, AND A COMPLEXING AGENT FOR CUPRIC IONS, SAID PROCESS BEING CHARECTERIZED BY THE IMPROVEMENT ACCORDING TO WHICH THE TEMPERATURE OF THE BATH IS MAINTAINED CONTINUOUSLY BETWEEN 70*C AND 80*C AND THE BATH IS ADJUSTED TO MAINTAIN CONTINUOUSLY A CYANIDE ION CONCENTRATION WITHIN THE RANGE OF 0.0002 TO 0.0004 MOLAR.
 2. A process as claimed in claim 1 wherein the temperature is maintained within one half degree of 73*C.
 3. A process as claimed in claim 1 wherein the cyanide ion concentration is maintained very close to 0.00025 molar.
 4. A process as claimed in claim 1 wherein the reducing agent is formaldehyde.
 5. A process as claimed in claim 1 wherein the complexing agent is ethylenediamine tetraacetic acid.
 6. A process as claimed in claim 1 wherein the surfactant is an organic phosphate ester.
 7. A process as claimed in claim 1 wherein the pH is maintained within the range 11.6 to 11.8. 